It has been recognised that certain properties of carrier fluids for liquid developers are required for effective functioning in conventional electrostatographic liquid development processes. Many of the physical requirements are mandatory, as known by those skilled in the art, but there are also other considerations, such as low toxicity, fire safety, low solvent power, low odour etc. For these reasons, isoparaffinic-hydrocarbons such as the Isopar range manufactured by Exxon Corporation, the Shellsol range manufactured by Shell Chemical and the Soltrol range manufactured by Phillips Petroleum became the industry standards for liquid toner carriers.
In more recent times, however, increased environmental concerns have placed liquid development processes under increasing pressure to further reduce or eliminate volatile emissions.
Other carrier materials applicable to liquid developers have been investigated and of these, silicone fluids are clearly liquids which combine all previously and currently desired properties of a modem liquid toner carrier.
In general, a liquid toner for developing electrostatic images is prepared by dispersing an inorganic or organic colorant such as iron oxide, carbon black, nigrosine, phthalocyanine blue, benzidine yellow, quinacridone pink and the like into a liquid vehicle which may contain dissolved or dispersed therein synthetic or naturally occurring polymers such as acrylics and their copolymers, alkyds, rosins, rosin esters, epoxies, polyvinyl acetate, styrene-butadiene, cyclised rubber, ethylene vinyl acetate copolymers, polyethylene, etc. Additionally, to impart or enhance an electrostatic charge on such dispersed particles, additives known as charge directors or charge control agents may be included. Such materials can be metallic soaps, fatty acids, lecithin, and the like.
Similarly an area of great interest exists in the development of liquid compositions for ink jet printing processes that employ environmentally friendly fluids as the carrier liquid. There are problems, however, with dispersion of marking particles such as organic pigments into such carrier liquids.
Silicone fluids have been used as carriers for liquid toners, eg. in U.S. Pat. No. 3,105,821 to S. W. Johnson, and U.S. Pat. No. 3,053,688 to H. G. Greig. Both of these early patents recognised the virtues of silicone fluids, but the understanding of the functioning of liquid toners at that time was relatively empirical, with those patents teaching simply the mechanical dispersion of a dry toner into the silicone fluid with no regard to chemical compatibility, which in turn governs the final particle size and stability of the dispersion so produced. More recently silicone fluids have again been recognised as desirable carrier fluids for liquid toners.
However, in the above application reliance is also placed on mechanical dispersion only. It is well known that silicone fluids have low solvent power for plastics and this property is well suited for copy machine components and organic photoconductor life. An unfortunate corollary to this is that many dispersion agents normally used in liquid toners, are incompatible with silicone fluids.
U.S. Pat. No. 5,612,162 to Lawson et al, as well as U.S. Pat. No. 5,591,557 to Lawson et al disclose compositions and methods of forming a liquid developer in silicone fluid. The teachings of these patents and formulations therein suffer from inadequate dispersion quality as required by high quality electrostatic printing. In particular, silicone fluid compatible dispersing agents are not employed in these formulations and consequently rivulet formation is a problem not addressed by these patents; such rivulets are manifest as disruptive localised areas of the continuous image and are similar to patterns observed when a high viscosity material is applied to flat surfaces as a thin film by means of a roller applicator.
It has been found that the particle size obtainable and the stability of dispersions of the prior art, have been inadequate due to the inability of the silicone fluid to fully disperse the marking particles sufficiently to achieve and maintain the required particle size during manufacture and use.
To further alleviate the environmental concerns as disclosed earlier, the concept of using high viscosity carrier fluids and or high solids content of the marking particles as liquid developers has been proposed. This type of liquid toner can develop an electrostatic latent image by use of a thin film of such highly concentrated liquid toners, within a process whereby these toners adhere selectively to the image part of an electrostatic latent image on an image bearing member without the toner adhering to the non-image part. This liquid developing method occurs by preferential adherence to the electrostatic latent image carrier surface under the dominant influence of the electric field strength of the electrostatic latent image, the quantity of toner transferred being proportional to the relative incremental field strength of the latent electrostatic image. This is a very high speed development method, in comparison to conventional liquid development which is solely reliant on electrophoretic migration along relatively large development gaps. An electrostatic printer utilising this type of high viscosity toner is described in Patent specification WO095/08792. With this concept however, a number of problems arise in relation to the liquid developer in such printing systems.
Such problems as the dispersion quality of the marking particles are apparent in printed copies from such non-impact printing devices using such liquid developers and are manifest as characters or areas of low optical density or poorly defined resolution with between such characters excessive background fog or background noise. The low optical density in many cases is related to the formation of so called rivulets.
This invention proposes an improvement in the dispersion quality of the marking particles thereby alleviating the problems noted above. In particular the problem of rivulet formation which is thought to be caused by the inadequate dispersion of marking particles such as pigments in a liquid developer is alleviated.
Such rivulet formation as so described can be readily seen in prints from a non-impact printing device such as an electrostatic printer of the type described in Patent specification WO095/08792. Although it must also be understood that such rivulet formation has been seen to detract from prints from other printing devices.
Thus the need exists for a method of minimising this rivulet formation by the inclusion of appropriate dispersion additives into preferred, high viscosity, high solids content liquid developers comprising liquid carriers which meet modem environmental demands and produce high quality prints with improved image resolution and higher optical density through the action of maintaining improved dispersion quality of the marking particles within the carrier liquid.
It is the object of this invention therefore, to provide an improved liquid toner composition which has additives which result in improved dispersion quality and thereby eliminating the formation of rivulets.
It is a further object of this invention to provide a liquid toner composition which has additives which result in improved particle size distribution, due to improved dispersion, as a result of increased steric stabilisation of the marking particles during manufacture, therefore inhibiting agglomeration during manufacture and for the life of the liquid developer.
It is a further object of this invention to provide a liquid toner composition which has additives which result in improved stability over a range of climatic conditions, and therefore having greatly improved shelf-life.
The liquid toner composition of this invention may be an ink for use in inkjet type printers, it may be a toner or liquid developer for electrostatography.